A central problem in immunology is how the immune system launches robust immunity against invading pathogens, while maintaining tolerance to self. This problem assumes a particular significance in the intestine because of the trillions of commensal microorganisms and food antigens that confront the intestinal immune system every day. Recent advances suggest that dendritic cells (DCs) play a fundamental role in maintaining the balance between immunity and tolerance. We now know that there are multiple subpopulations of DCs that differentially regulate the immune response, and that these subsets display tremendous functional plasticity in response to instructive signals from microbes and microenvironments. In this context, our recent work suggests that DCs and macrophages in the lamina propria of the intestine differentially regulate Th17 versus T regulatory responses, and that a dynamic interplay between DCs and macrophages maintains a balance between immunity and tolerance. However, there are multiple subpopulations of DCs in the lamina propria. This raises several fundamental questions: (i) What roles do the distinct DC subsets play in regulating immunity versus tolerance? (ii) Are the functions of these cells fixed, or are they plastic? (iii) To what extent do commensals shape the function of DCs in the intestine? (iv) How can oral vaccines target intestinal DCs to stimulate optimally effective mucosal immunity? The present grant will address these questions in the following specific aims: Aim 1: To determine whether distinct subsets of lamina propria DCs and macrophages differentially bias the class of innate and adaptive immune responses Aim 2: To determine whether commensal bacterial flora regulate the functions of lamina propria DCs and macrophages and their ability to induce Th17 versus T regulatory responses Aim 3 : To determine the innate responses of lamina propria DCs and macrophages to oral administration of adjuvants or vaccines, and the effects of such responses on the adaptive immune response The successful completion of these aims will provide fundamental mechanistic insights into how the immune system maintains a balance between immunity and tolerance, and provide new strategies for modulating mucosal immunity to control autoimmunity, or to protect against oral infections. Public Health Relevance: The immune system has the dichotomous property of being able to launch effective immunity against pathogens, while being tolerant to host antigens. This property is particularly important in the intestine, where the immune system is confronted with trillions of commensal bacteria and food antigens, which must be tolerated, whilst it remains vigilant for harmful intestinal pathogens. Although this property has been appreciated for over a century, the mechanisms by which it occurs remains poorly defined. Recently we have identified a macrophage cell type in the intestine, which suppresses inflammation. In contrast, a different cell type known as the dendritic cell seems to promote inflammation. The focus of the present proposal is to understand how these cell types regulate the decision making process with respect to immunity versus tolerance in the intestine.